Millar is an eleven-time winner of the Canadian Show Jumping Championship[3] and has captured six Spruce Meadows Derbys, having amassed $3.5 million in prize earnings there.

With his horse, Big Ben (1976–1999), Millar won more than 40 Grand Prix titles worldwide and the Show Jumping World Cup two years in a row. At the Pan American Games in August 1987, Ian Millar became the second Canadian to win an individual Pan Am gold medal. He holds the North American record for Grand Prix and Derby wins. He has been a member of every Canadian Equestrian Team at the Show Jumping World Championships team since 1972. At the 2008 Summer Olympics, Millar competed at his ninth games, tying the record set by Hubert Raudaschl. He has been named to ten straight Olympic teams, but did not compete at the 1980 Summer Olympics because of the Canadian boycott.[4]

On August 18, at the 2008 Summer Olympics in Beijing, at the age of 61, Millar anchored his team (also including Jill Henselwood, Eric Lamaze, and Mac Cone) to a first-place standing. Riding In Style, he completed a faultless run to lead the Canadian team into a jump-off for gold with the United States. In the team equestrian gold-medal jump-off, Canada won the silver medal, Millar's first Olympic medal.[5]

Through 2011, Millar continued to win in the Grand Prix ring. He said that his next goal was to be selected for the 2012 Summer Olympics in London, hoping to be joined on the Canadian team by son Jonathan, then 36, and daughter Amy, then 34. Millar was named to the 2012 Olympic team, making that appearance, his tenth, a record for any Olympic athlete's appearances at Olympics.[6] In the 2012 Olympics' Individual Jumping event, Millar finished in a three-way tie for 9th, the best Canadian result. In Team Jumping, Millar, along with fellow riders Jill Henselwood and Eric Lamaze, scored a fifth place finish for Canada.

On September 14, 2014, Millar won the $1.5-million CP International at Spruce Meadows aboard Dixson, who shares bloodlines with Big Ben. [7]